Cupola melting of cast iron



Patented June 23, 1953 CUPOLA MELTING OF CAST IRON Sam F. Carter, J r.,Birmingham, Ala., assignor to American Cast Iron Pipe Company,Birmingham, Ala., a corporation of Georgia No Drawing. Application April27, 1950,

Serial No. 158,607

7 Claims.

This invention relates to the manufacture of cast iron, and moreparticularly to an improved process for melting cast iron in a cupola.

Dne of the principal limitations of conventional methods of cupolamelting of iron is the sulfur increase resulting from absorption fromthe coke, a condition which limits the amounts of scrap and lessexpensive materials usable in the cupola charge and which frequentlynecessitates ladle desulfurization in order to maintain the sulfurcontent within specified limits. Another characteristic of the ordinarycupola procedure which places quality and economic restrictions on itsuse is that it does not decrease, and in some instances may increase,the phosphorus content of the metal charge, a fact which limits theextent to which the less expensive, high phosphorus irons can be used. Afurther limitation on the utility of conventional cupola meltingprocesses resides in their inability to increase the carbon pick-up ofthe iron beyond a certain limited extent dependent upon the carboncontent of the coke and metal charge, the result being that, when a,high carbon iron is required, it is frequently necessary to use specialmaterials such as lump graphite, pitch coke, special briquets and thelike.

In addition to the more obvious advantages of cast irons which are lowin sulfur and phosphorus and high in carbon, recent developments in theproduction of nodular graphite iron have placed further emphasis on theimportance of the sulfur, phosphorus and carbon contents. Better alloyrecovery and better control are obtained and the costly alloy is notconsumed in desulfurizing, if the sulfur is low originally, whileductility advantages are realized more fully with a low phosphoruscontent. Since irons chemically suitable for nodular treatment currentlyrequire either electric furnace melting, large scale ladledesulfurization previous to alloy addition, or excessive andunpredictable loss of the special alloy, a less expensive, more directmethod of obtaining molten metal of the desired characteristics wouldcontribute materially to more economical production and greater use ofnodular irons.

It is therefore one of the principal objects of the present invention toprovide an improved process for melting cast iron in a cupola wherebylow sulfur, low phosphorus, high carbon cast iron can be economicallyproduced in a single melting operation.

Another object is to provide a novel procedure for the cupola melting ofiron which will enable the production of types of iron heretoforeunproducible in a cupola and will permit economies by the use in thecharge of cheaper metals previously limited in their utility because oftheir unfavorable chemistry.

A further object of the invention is to provide a method for meltingcast iron in a cupola which is characterized by a higher meltingtemperature and a faster rate of melting than could be attained byoperating the same cupola in the usual manner.

Still another object is to provide a cupola melting process whichgreatly increases the carbon pick-up of the iron in comparison withconventional procedures, and thus permits more economic production oflow phosphorus iron from large proportions of steel scrap.

A still further object is to provide a process which is effective todesulfurize cast iron while it is being melted in a cupola, therebyavoiding the necessity for, and disadvantages incident to, subsequentdesulfurizatlon treatment in the ladle.

Another object is to provide an improved process for the cupola meltingof cast iron which produces irons of high quality, including thosesuitable for nodular graphite treatment, directly and economically fromstarting materials having only a limited utility in ordinary cupolaoperations.

These and other objects, including minimization of losses of oxidizableelements in the cupola char e and the reduction of some elements fromtheir oxides, will appear more fully as the description of the inventionproceeds.

I have discovered that the addition of a relatively small amount ofcalcium carbide to the cupola charge is effective to produce results andadvantages not heretofore attainable by conven tional cupola meltingprocedures and is superior, particularly from the standpoint of economy,to the various special treatments which have been suggested for thepurpose of attaining results similar to those obtained by the presentinvention.

Although it has long been recognized that improved desulfurizationefficiency can be attained in the making of steel when a smallconcentration of calcium carbide is formed in, or added to, the slag,the practices employed in removing sulfur from steel are not applicableto the cupola melting of cast iron because the cupola, being acontinuous, direct melting, shaft type furnace, has no open loath ofmolten metal which would permit manipulation of the slag. It has alsobeen proposed to use calcium carbide as a desulfurizing agent by addinit directly to molten cast iron in the ladle, but this ladle treatmentnot only increases the cost of manufacture, but requires additionalhandling and produces temperature losses and danger of slag inclusions,all of which are objectionable features. As far as I aware, calciumcarbide has never been used as a part of the cupola charge in the mannercontemplated by the present invention.

In carrying out the process of the presentinvention, calcium carbide,preferably in the form of lumps from inch to 3 inches in size, is mixedwith the coke of the cupola charge in an amount such that the carbideconstitutes between about 1% and 7% by weight of the metal portion ofthe charge. The amount of calcium carbide required for any particularcharge will depend primarily upon the character of the metal, coke andother flux being used and the desired chemistry of the final metal, butmay also be influenced by such other conditions as the desiredtemperature and rate of melting. Under ordinary or normal conditionsofravv material quality and metal specification, from 2% to 4% calciumcarbide will usually be sufficient for all practical purposes. However,when extremely low sulfurs, high carbons .or high temperatures aredesired, or when unusually high sulfur starting materials are used, from4% to 7% calcium carbide may be necessary. On the other .hand, when themetal charge is low in sulfur, as in the case of an all pig iron charge,from 1% to 3% carbide will suffice inmost cases, A tabular summary ofthe conditions and results of a number of specific heats will be givenhereinafter to indicate in detail how the invention may be applied inpractice to various charge materials for the. attainment of variousresults.

Although the carbide can be added with each individual coke charge, ithas been found that maximum effectiveness is not realized untilthecarbide reaches. the bedunder the charges being melted, therebyproducing a' lag of approximately ten charges, depending upon the sizeof the charges inrelation to the cupola diameter, the melting rate, etc.For example, in operating a 23 inch diameter cupola with 100 lb. metalcharges, and adding the calcium carbide with each charge, the maximumtemperature and highest carbon and lowest sulfur levels were reachedafter from eight to ten charges had been melted, equivalent to 30minutes melting time or about 30-40 inches of charge descent.Accordingly, in order to compensate for this lag and to obtain themaximum effects with optimum uniformity, it is preferable to omit thecarbide from the lower half of the bed, say' for a distance of about 20inches above the t'uyeres, to distribute it uniformly throughout thecoke in the upper half of the bed, and then add it with each coke chargeor split until about the tenth char e from the last, i. e., untilapproximately 30 minutes from the end of the heat.

While the process of the present invention is applicable to any cupolaor other type of direct melting, shaft type furnace, all of the improvedresults and advantages thereof are realized most completely when theprocess is carried out in a basic lined cupola wherein slags of highbasicity can be maintained. For example, a cupola lined with dead burnedmagnesite brick laid with magnesite mortar has been found to produceexcellent results. Dolomite refractories were equally successfulchemically, although somewhat in- 'ing over 3.20% carbon from all steelcharges.

ferior to magnesite from a refractory standpoint. Heats run in aconventional acid lined cupola produced the same effects as thoseattained in the basic lined cupola, but to a less degree.

Whereas cupola melting of cast iron in the usual manner normallyproducesan increase of about .05% in the sulfur content of the molten iron incomparison with that of the metal charge, because of absorption ofsulfur from the coke, the use of calcium carbide in the cupola charge inaccordance with the present invention has removed as much as 187% sulfurfrom a high sulfur scrap charge which would be otherwise worthless. Withmetal charges of conventional character, theaddition of carbide hasproduced cast irons having a sulfur content only a fraction as high asthat resulting from normal cupola melting procedures. As an example ofthe extent to which desulfurization can be carriedby the presentprocess, it will appear from the data hereinafter set forth that castiron having a sulfur content as low as 009% has been produced by the useof calcium carbide in the cupola charge, a result heretofore attainableonly by the most expensive basic electric process. In the abovementioned case where 187% sulfur was removed, 5% calcium carbide wasadded to the coke with an all scrap charge having an initial sulfurcontent of 287%, while the .009% sulfur content resulted from an all pigiron charge and a 6% addition of calcium carbide. Such desulfurizationefficiency permits the use of more high sulfur scrap by foundries andextends the field of utility of the cupola in duplex steel making.

In addition to highly efiicient desulfuri'zation, another advantage ofthe use of calciumcarbide in the cupola melting of cast iron which is ofgreat economic importance is the substantially increased carbon pick-upwhich results therefrom and which in turn provides indirectly theadditional effect of lowering the-phosphorus content, because increasedcarbon absorption permits the use of larger proportions of lessexpensive, low phosphorus steel scrap with the attendant considerablesaving in cost. As will also appear hereinafter, the increase in carbonpick-up incident to the use of calcium carbide in the cupola charge issufficient to produce cast iron contain- As an example of th economicimportance of the phosphorus content, the following figures representthe comparative costs per gross ton, including freight, of pig iron,cast iron scrap and steel scrap of different phosphorus contents asdelivered at the same foundry:

Phosphorus Total Cost Content For Ton Percent Low phosphorus pig iron 05$63. 50 Medium phosphorus pig iron 20 55.00 Southern high phosphorus pigiro 39. 00 Cast iron scrap 20. 37. 00 Steel scrap 05 32. 65

cupola. The comparative metal costs were as follows:

Heat I.Basic lined cupola with 3% calcium calrbtde added [Sulfur .026%melting temperature 2,900 F.]'

Heat H.-Conventional cupola [Sulfur .090%--melting temperature 2,750 F.]

80% low phosphorus pig (1.50% Si), 1,600 lbs .at

$63.50 per ton $4AO 20% steel scrap, 400 lbs. at $32.65 per ton p.83

1.40%1s ilicon 3.375% FeSi, 37.4 lbs. at 10% cents per Heat III.-C'onventio1ml cupola [Sulfur .07 0%melting temperature 2,700" R] 100low phosphorus pig 1.50% s1 2,000 lbs. at 3.50 per ton 1.10%1gi1icon as75% FeSi, 30 lbs. at 10% cents Per As will be seen from these figures,after adding the cost of the'calcium carbide, additional silicon, basiclining, fluxes, etc., the all steel-calcium carbide heat according tothe present process had a cost advantage of approximately $12.00 net perton of metal over the currently conventional charges and procedure. Ifthe temperatures and sulfur contents of Heats II and III were adjustedto meet those of Heat I, the cost advantage of the latter would be evengreater. For example, to lower the sulfur content of Heat II from .090%to 026% would require a ladle treatment costing from $1.00 to $3.00 perton, while to deliver the cast iron at the same temperature afterdesulfurization would involve the additional cost of producing a 200 F.increase in temperature.

The production of an increased melting temperature is another advantageof the process of the present invention, particularly in comparison withthe net temperature loss which results when ladle desulfurization isnecessary of molten cast iron produced in accordance with conventionalcupola practice. Although it was well known that to increase the amountof ordinary flux in 6 the cupola charge would effect a substantialdecrease in the melting temperature, it has been found that the additionof calcium carbide to the charge brings about a material increase in themelting temperature, in addition to the other reactions produced. Forexample, whereas an increase in the amount of ordinary flux by 7% (basedon the weight of the iron charge) normally decreases the meltingtemperature from 30 to F., the addition of from 5% .to 7% of calciumcarbide, likewise based on the weight of the metal charge, produces anincrease in the melting temperature of over F. The temperature increasesobtained by the use of calcium carbide result in temperatures well abovethe maximum normally obtainable in the same cupola even under optimumconditions of conventional operation, and are attained without requiringstructural modification of the cupola or subsequent heating in a bathtype furnace, expedients that would ordinarily be required in order toreach the same temperature without the use of calcium carbide.

Below the maximum temperature limit, increases in melting temperature inthe conventional cupola normally require increased quantities of fueland a sacrifice of output.v On the contrary, the addition of smallpercentages of calcium carbide to the charge produces an increasedtemperature with an actual decrease in quantity of fuel required and anincrease in output. For example, a 7% addition of calcium carbide witheach coke charge permits reduction of the coke from 17% of the metalcharge to r from 4 to 6% thereof, while at the same time producing a 20F. higher temperature and a 40% increase in melting rate.-

In order to indicate the scope and manner of practical application ofthe present invention, the following tabulation sets forth theconditions and results of a number of typical heats wherein cast ironwas melted in a basic lined cupola in the presence of a small amount ofcalcium carbide. In this table, the percentages of carbide, flux andcoke are all based on the weight of the metal charge, while the figuresgiven as to the amounts of sulfur, carbon, silicon, manganese andphosphorus in the charge and in the final product are the averages offrom three to seven analytical determinations.

. Flux Temperature (F.)

- Calcium Heat No. Metal Charge Carbide Coke Max Limestone FluorsparLeve'l Convent.

Percent Percent High sulfur remelt 5%- (Charge) 5 2 14% f un ry 740 720do- -6%. Bed 5 3 .-...do 2,815 2.720 5 3 2, 780 2, 720 5 3 2, 935 2, 8005 3 2, 920 2, 800 5 3 2, 880 2, 800 5 2 2, 900 2, 750 5' 2 2, 920 2, 7505 2 2, 730 2, 700 5 3 2, 780 2, 700 1 2, 870 2, 750

1 12 high sulfur 2, 880

l 9% hi h sulfur. 2, 900

1* 6% high sulfur 2, 880

l 4% foundry 2, 820

1 No'adequate basis available for estimating.

(Charge) indicates that all of the calcium carbide was added with theindividual coke charges.

(Bed) indicates that some of the calcium carbide was distributed withthe coke in the top half of the bed and the remainder was added with thecoke charges until ten charges from the end of the heat.

Maximum temperature level was the stream temperature averaged for threeoptical pyrometer readings after overcoming the-chilling'efiectsota coldtrough, etc.

Convent. indicates the approximate temperature that would be expectedfrom the same charge, omitting the calcium carbide, melted in aconventional acidlincd cupola, based on experien cc and the literature.

H Percent Sulfur Content Percent Carbon Content Percent Other Elements(Final) at I Charge Final Convent. Charge Final Convent. SiliconManganese Phosphorus 287 100 250+ 2. 20 2. 89 2. 70 1. 79 81 25 134 016159 2. 30 3. 83 2. 80 l. 85 72 03 119 031 140 2. 48 3.62 3.00 2.31 89.03 050 O25 140 20 3. 52 2. 20 2. 67 88 02 050 .026 140 20 3. 50 2. 202. 41 .80 .03 050 047 140 20 3. 19 I 2. 20 2. 40 S 02 .060 .032 .230 1.3. 30 2. 40 2.13 .74 22 .060 .037 230 1. 20 3. 65 f 2. 40 2.10 .68 04.030 010 070 4. 20 4.17 1 3.70 2.37 ll L03 .030 009 070 4. 20 4. 23 3.70 1. 98 14 04 .060 059 .230 1. 20 3. 30 2. 40 2.06 74 31 .060 111 1. 202. 64 1.98 .72 .31 060 116 1. 20 2. 60 1. 86 71 26 060 .172 1. 20 2.39 1. 99 66 .31 060 .085 l. 20 2. 75 2.03 v 71 .19

1 N o adequate basis available for estimating.

Convent. indicates the approximate analysis that would be expected fromthe same charge, ornitting the calcium carbide, melted in a conventionalacid lined cupola, based on experience and the literature.

The properties of the two types of coke used in the heats of the abovetabulation were as follows:

In heats 1, 2 and 3 of the table, high sulfur scrap charges were meltedwhich ordinarily could not be used in conventional cupola operations.The low sulfur contents obtained indicate the powerful desulfurizingcapacity of the calcium carbide when added within the cupola. Heats 4, 5and 6, wherein all steel charges were melted, illustrate the greatlyincreased carbon pick-up combined with desulfurization which provides anew method of producing a low phosphorus, low sulfur iron. In heats 7and 8, 70% steel charges were melted with a high sulfur coke which wouldnormally produce an iron having over 230% sulfur and less than 2.40%carbon, and would therefore be considered unsuitable for cupola use.However, with aid of a 6% addition or" calcium carbide in these twoheats,

very low sulfurs of 032% and 037% were obtained along with sufficientlyhigh carbons of 3.30% and 3.65%. Heats 9 and 10, using charges of allpig iron, produced the extremely low final sulfur contents of .010% and.009% directly from the cupola, as effective a desulfurization as couldbe obtained by any of the more expensive meltan example of a successfulmelt made with a 7% carbide addition and a foundry coke'charge amountingto 'only"4% of the metal charge. In the latter instance, the temperatureand chemistry were comparable with, and the output was greater than, theresults that would have been obtained using 15-20% coke without thecarbide.

As indicated in the foregoing tabulation, in some of the heats all ofthe calcium carbide was added with the individual coke charges, while inother heats the carbide was uniformly distributed with the-coke in thetop half of the bed and then added with. each coke charge until aboutten charges from the end of the heat. For example, in heat 5, after 130lbs. of the bed coke had been laid, 2 lbs. of calcium carbide were addedwith each 16 lbs. of coke for eighteen buckets to comvplete the bed.After lighting off and burning have produced the same results in kind,but not to the same degree, as those realized in the basic lined cupola.For example, 70% steel, 30% pig iron charge melted in an acid linedcupola with a 4% addition ofcarbide experienced a .100%

sulfur instead of .130%, a 2.82% carbon instead of 2.50%, and atemperature increase from an average of 2780 to 2860 F.

The reactions resulting from introduction of calcium carbide into thecupola charge which account for thesimultaneous temperature increase,--sulfur reduction and carbon pick-up are not yet fully understood.Whether the reaction ing methods. Heats 11, 12. 13 and 14, using a 7%carbon contents are not of paramount importance, greater utilization canbe made of the thermal advantages of calcium carbide by substituting itfor a portion of the coke. Heat 15 is takes place from continual contactof molten metal with carbide in the basin of the cupola, from the localreducing effect of hydrocarbon gases produced from th decomposition ofthe carbide from atmospheric moisture, from contact of metal drops withthe unmelted carbide, or from a surface reaction between the carbide andthe iron oxide in the slag drops, is debatable. It is also possible, ofcourse, that a combination of several reactions may occur. However, thedelayed'effect and the presence of some unconsumed carbide in the bottomdrop of some heat has led to the conclusion that the carbide isnotimmediately melted or decomposed, a conclusion which is also supportedby the fact that locating all of the carbidein the;cup01a' basinproduced some of all of the. effects obtainable by the useof carbidabutnot to the extent experienced when all the carbide was distributedthroughout the coke bed, and the further fact that carbide briquets haveproved less effective than bare lump carbide.

, Frorn'the experiences encountered in practic-- ing the process of thepresent invention, it wouldseem that the principal'eiiect derived fromthe use of calcium carbide results from a reaction at the surface of-thecarbide with the molten iron and slag, a reaction which seems moreactive at the higher temperatures obtained. in the melting zone.Apparently, metal drops impinging on the surfaceof the carbide lumps ata temperatureof from 2000 to 3000 F. absorb carbon from the carbidewhile the carbide absorbs sulfur'from the metal. The reactionappears tobe exothermic and thereby superheats the metal drops. The decompositionproduct of the carbide, calcium oxide, would then contribute to slagbasicity and retain as calcium sulfide the sulfur absorbed from themetal. It also seems likely that slag drops impinging on the surface ofthe carbide produce a reaction reducing the iron and other oxides, andalso absorb the calcium oxide from the decomposed carbide, and possiblysome calcium carbide. Such a slag, with increased basicity, decreasediron oxide and some residual calcium carbide, would have considerablecapacity to absorb sulfur during its further descent and aftercontacting the metal in the basin of the cupola.

The experience with coke reductions suggests that carbide might simplyburn in the presence of air if sufficiently heated and if initiated orcatalyzed by one of these other reactions. Of course, any carbideprematurely decomposed by reaction with moisture would produce the veryreducing acetylene gas and basic calcium oxide with conceivably similarresults. Likewise, any unoonsumed carbide that might reach the basincould still continue its effect, but with decreased efficiency.

Whatever may be the nature of the reactions in question, it has beenestablished that the addition of small percentages of calcium carbide tothe cupola charge results in improved desulfurization, greater carbonpick-up and an increased melting temperature, all of which effects areof decided advantage and benefit in the cupola melting of cast iron.

Other results of the use of calcium carbide in accordance with thepresent invention which may be of advantage in special applicationsinclude the production of a more thoroughly deoxidized iron, decreasedloss of oxidizable elements, and the reduction of some elements fromtheir oxides. It has been suggested that variations in the residual ironoxide content of cast iron have a considerable effect on graphitizationand subsequent properties of the iron, and various special reducingmaterials have been offered commercially for the purpose of producing amore thoroughly deoxidized iron from the cupola. The decreased amount ofiron oxide in the slag produced by the present process indicates thatcalcium carbide is far more powerful in deoxidation effectiveness thanany of the special reducing materials now available. As indicative ofthe effect of the present invention in minimizing losses of oxidizableelements in the cupola charge,

10 it is noted that the heats represented by the foregoing tabulation,particularly those numbered 2-1 0, experienced considerably lower lossesof silicon and manganese than comparable heats made without the use ofcalcium carbide, and only half the losses in many cases, a result whichisconsistent with the more thoroughly deoxi-v dized iron and the lowerslag iron oxide. In addition to the economic advantage resultingfromreduced consumption of siliconand manganese, the decreased loss ofoxidizable elements incident to the present process points to thepossibility of using some of the more readily oxi diz-able alloys inthecupola which ordinarily experience excessive loss. That the strongreducing action of the calcium carbide may be utilized in the cupola orin other shaft type furnaces, such as the blast furnace, to deliberatelyreduce certain alloys from their oxides is suggested by the fact that asmuch as .005% magnesium, which is one of the more difiicult elements toreduce, has been obtained in iron melted in accordance with the presentinvention by chemical reduction of the magnesium oxide refractories. Itis also possible that, with the aid of carbide, the reduction of certainmetal oxides might be obtained either in furnaces of simplerconstruction, at a faster rate, or with greater efficiency.

There is thus provided by the present invention an improved process forthe cupola melting of cast iron which produces benefits and advantagesunattainable by the various procedures previously practiced, and whichmakes possible substantial savings in cost because of the capacity ofthe process to function with relatively low cost starting materialshaving only a limited utility in conventional cupola operations. Amongthe more important results flowing from addition of relatively smallamounts of calcium carbide to the cupola charge in accordance with theinvention are the following: greatly increased carbon pick-up whichpermits more economic production of low phosphorus iron from largeproportions of steel scrap; desulfurization directly within the cupolasufficient to permit economic reclamation of low cost, high sulfurmaterials; increased melting temperature beyond the maximum attainablein the conventional cupola, and increased melting rate; decreased lossof oxidizable elements, reduction of some elements from their oxides anda more thoroughly deoxidized cupola iron. It will be evident that thepresent invention thus materially extends the field of utility of cupolamelting of iron and makes it possible to obtain in the cupola resultswhich differ in both kind and degree from those heretofore realized.

Although a number of specific examples have been given herein of themanner in which the invention may be carried out in practice, it will beunderstood that these examples are illustrative only and are notintended to represent the full scope of the inventive concept. Referenceis therefore to be had to the appended claims for a definition of thelimits of the invention.

What is claimed is:

l. The method of melting iron metal which comprises the steps ofintroducing into a shaft type furnace above the melting zone thereof acharge containing coke, iron metal and calcium carbide in solid form,and melting the metal portion of the charge by combustion of the coke inthe presence of the calcium carbide so that the drops of molten metaland slag contact di- 11 rectly with solid carbide surfaces in themelting zone.

2. The method of claim 1 wherein the amount of calcium carbide in thecharge is from 1% to 7% by weight of the metal portion of the charge.

3. A process of producing low sulfur, high carbon cast iron directlyfrom a cupola, without subsequent treatment, which comprises the stepsof introducing into a cupola at a point substantially above the meltingzone thereof charges containing iron metal, coke and calcium carbide insolid form, melting the metal portion of the charges by combustion ofthe coke in the presence of the calcium carbide, and utilizing theproducts of combustion to heat the carbide during descent of the chargesto the melting zone.

4. The process of claim 3 wherein the calcium carbide is mixed with thecoke portion of the charges and constitutes from 1% to 7% by weight ofthe metal portion of the charges.

5. The process of claim 3 wherein the metal portion of the charges has asulfur content in excess of .1%.

12 6. The process of claim 3 wherein the metal portion of the chargescontains over 50% steel. 7. The process of claim 3 wherein the cupola isprovided with a basic lining.

Published by the Western Society of Engineers, Chicago, Illinois.

3. A PROCESS OF PRODUCING LOW SULFUR, HIGH CARBON CAST IRON DIRECTLYFROM A CUPOLA, WITHOUT SUBSEQUENT TREATMENT, WHICH COMPRISES THE STEPSOF INTRODUCING INTO A CUPOLA AT A POINT SUBSTANTIALLY ABOVE THE MEETINGZONE THEREOF CHARGES CONTAINING IRON METAL, COKE AND CALCIUM CARBIDE INSOLID FORM, MELTING THE METAL PORTION OF THE CHARGES BY COMBUSTION OFTHE COKE IN THE PRESENCE OF THE CALCIUM CARBIDE, AND UTILIZING THEPRODUCTS OF COMBUSTION TO HEAT THE CARBIDE DURING DESCENT OF THE CHARGESTO THE MELTING ZONE.